The ligand of 6-(4-benzenesulfonic acid azo)-3-aminophenol (HL) is prepared and 
characterized by elemental analyses, IR, 1 H NMR spectra, mass spectra and X-ray 
diffraction analysis. The proton–ligand dissociation constants of HL ligand and its metal 
stability constants with Mn (II), Co (II), Ni (II) and Cu (II) ions have been determined using 
potentiometric studies as described by Irving and Rossotti. The optimized structure and the 
calculated quantum chemical parameters of HL ligand were studied. The potentiometric